Address de-registration from IP multimedia networks

ABSTRACT

According to one aspect, a communication network system comprising at least one first network and at least one second network providing application services over the first network is described. The system comprises first network means (GGSN) for receiving a message associated with an IP address allocated by the first network from the second network, for detecting that there is no active communication channel for this IP address which is allocated to a user equipment, and for sending an indication that the user equipment is not reachable through this IP address to the second network. The system further comprises second network means (P-CSCF/PCF) for receiving this indication.

FIELD OF THE INVENTION

The present invention relates to the de-registration of IP addressesassigned to users in an access network like GPRS (General Packet RadioService) and IP (Internet Protocol) Multimedia networks.

BACKGROUND OF THE INVENTION

The future VoIP (Voice over IP) networks are being designed using thelayered approach, consisting of IM (IP Multimedia) networks providingthe Application Level services over some Transport Level access networkwith maximum possible independence between the two layers. The 3GPPstandardization efforts for R00 were focused on GPRS as the TransportLevel access network.

In order to access the IP Multimedia services via the GPRS accessnetwork, the UE (UE refers to User Equipment or Application; whenever UEor User Equipment is referred to later in this document, it may alsorefer to possible User Agent or Application) must perform GPRS attachand activate the necessary PDP (Packet Data Protocol) context first,before registering with the IM network. During the PDP contextactivation, the GPRS network (GGSN (Gateway GPRS Support Node), morespecifically) allocates to the UE an IP address, which is used by theGPRS network to address the UE. This IP address is conveyed to the IMnetwork during the application level registration and is used also bythe IM network to address the UE.

In most of the cases, the IP addresses are allocated to the userdynamically by the GPRS network during the PDP context activation. Thiscan be arranged by two ways: either a GGSN has a pool of IP addresses orthe DHCP (Dynamic Host Configuration Protocol) server allocates theaddress. An IP address thus allocated is freed when the PDP context isdeactivated explicitly by the UE performing PDP context deactivation orimplicitly, for example, after the UE becomes unreachable or fails toperform a periodic routing area update (RAU). The same IP address couldbe re-allocated to a different user by the GPRS network.

It may be possible that the UE explicitly performs PDP contextdeactivation with the GPRS network but does not perform any specificde-registration with the IM network. It is also possible that a PDPcontext associated with the UE is implicitly deactivated by the GPRSnetwork for some reason as mentioned above. In all these situations, theIM network is not aware that the IP address of the UE has becomeinvalid, or even worse, the IP address has been allocated to anotheruser. In the later case, messages coming from the IM network (e.g., incall setup) might be delivered to a different UE which happens to usethat IP address at present.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide consistency of IPaddresses assigned to users in an access network and networks providingapplication services through the access network.

According to one aspect of the present invention, this object isachieved by a communication network system comprising at least one firstnetwork (GPRS) and at least one second network providing applicationservices over the first network, the first network allocating an IPaddress used by the second network, said system comprising:

-   -   means for detecting that the IP address allocated to a user        equipment is de-allocated by the first network; and    -   means for initiating a de-registration of the de-allocated IP        address in the second network.

The system may further comprise a control entity (PCF) being arranged tocommunicate with a gating network element (GGSN) in the first networkand a dedicated call serving network element (CSCF) in the secondnetwork, the dedicated call serving network element serving therequested application,

-   -   wherein the gating network element detects a communication        channel deactivation, and informs the control entity thereof,    -   wherein the control entity detects that the deactivated        communication channel represents a signaling PDP context and        initiates the de-registration in the dedicated call serving        network element.

The gating network element may supply an indication of the signaling PDPcontext to the control entity.

The control entity may detect that the deactivated communication channelrepresents a signaling PDP context by checking request state informationin the control entity.

The gating network element may be informed by a serving network elementin the first network about a communication channel deactivation andforwards this information to the control entity.

Moreover, a network element in the first network may detect thede-allocation of the IP address and informs a server about thede-allocation, and the server initiates de-registration in call servingnetwork elements in the second network, to which said IP-address isassociated with.

The server may be a home subscriber server (HSS).

A network element (SGSN) in the first network may inform a database(HLR) about the de-allocation, the home database informs a mobilityserving entity (UMS) about the de-allocation, and the mobility servingentity initiates the de-registration in a dedicated call serving networkelement (CSCF) in the second network, the dedicated call serving networkelement serving the requested application.

Furthermore, the network element in the first network may detect adetach of the user equipment causing the de-allocation for all IPaddresses of the user equipment and inform the server about thede-allocation, and the server initiates de-registration in all callserving network elements of the user equipment.

A gating network element (GGSN) in the first network stores addresses ofcall serving network elements proposed to the user equipment against theallocated IP address and a user identification, the gating networkelement allocating the IP address and being involved in the discoveryprocedure of a call serving network element (CSCF) in the secondnetwork, the call serving network elements being able to serve therequested application, and, when the gating network element detects thede-allocation of the IP address, it initiates de-registration in allstored call serving network elements associated with the IP addresses inthe second network.

The user equipment may inform a gating network element (GGSN) in thefirst network whenever it registers with a call serving network element(CSCF) in the second network, the gating network element allocating theIP address and being involved in the discovery procedure of a callserving network element (CSCF) in the second network, the call servingnetwork elements being able to serve the requested application, whereinthe gating network element stores addresses of the registered callserving network elements against the allocated IP addresses and a useridentification, and, when the gating network element detects thede-allocation of the IP addresses, it initiates the de-registration inall stored call serving network nodes associated with the IP addresses.

The de-allocation of the IP address caused by a communication channeldeactivation may be detected by a gating network element in the firstnetwork, and the server is informed by the gating network element aboutthe de-allocation.

Moreover, a de-allocation of the IP address caused by a detach of theuser equipment is detected by a serving network element (SGSN) in thefirst network, and the home server is informed by the serving networkelement about the de-allocation.

The server may be informed by the user equipment about all call servingnetwork elements the user equipment is registered with.

The IP address may be de-registered in the second network by deleting arecord in a home server (HSS) in the first network by a call servingnetwork element in the second network.

An IP address may be allocated for each communication channel requestperformed by the user equipment.

The initiation of the de-registration may comprise the supply of ade-register message.

The first network may be a wireless access network and the secondnetwork may be an IP multimedia network.

In addition, the communication channel by be a PDP context.

Furthermore, the above-mentioned object is achieved by a control entitywhich is arranged to communicate with a gating network element (GGSN) ina first network and a dedicated call serving network element (CSCF) in asecond network providing application services over the first network,the first network allocating an IP address used by the second network,the dedicated call serving network element serving the requestedapplication,

-   -   wherein the control entity receives information from the gating        network element about a communication channel deactivation    -   wherein the control entity detects that the deactivated        communication channel represents a signaling PDP context and        initiates the de-registration in the dedicated call serving        network element.

Moreover, the above-mentioned object is achieved by a method ofproviding communication between at least one first network (GPRS) and atleast one second network providing application services over the firstnetwork, the first network allocating an IP address used by the secondnetwork, said method comprising:

-   -   detecting that the IP address allocated to a user equipment is        de-allocated by the first network; and    -   initiating a de-registration of the de-allocated IP address in        the second network.

The invention applies to cases when the UE explicitly or implicitlyde-registers with the access network and its IP address is freed. Ofcourse, after this has happened, the UE is not capable of communicatingwith the CSCF. For example, an explicit de-allocation comprises a PDPcontext deactivation or an explicit detach procedure when the UE isswitched off. An implicit de-allocation occurs, for example, when the UEfails to perform a periodic routing area update.

Users may be registered with multiple CSCFs of different IM networks,and the same user may have more than one IP addresses assigned.

The PCF functionality can be provided in a separate network element ormay be contained in another network element like CSCF.

Further features of the present invention are defined in the dependentclaims.

The invention applies to cases when the UE explicitly or implicitlyde-registers with the access network and its IP address is freed. Ofcourse, after this has happened, the UE is not capable of communicatingwith the CSCF. For example, an explicit de-allocation comprises a PDPcontext deactivation or an explicit detach procedure when the UE isswitched off. An implicit de-allocation occurs, for example, when the UEfails to perform a periodic routing area update.

In the following the present invention will be described by way ofembodiments thereof with reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a signaling diagram of an IP multimedia service access viaGPRS access network.

FIG. 2 shows a signaling diagram of a de-register procedure caused by anIP address de-allocation according to an embodiment of the presentinvention.

FIG. 3 shows a signaling diagram of a de-register procedure caused by aPDP context deactivation according to an embodiment of the presentinvention.

FIG. 4 shows a signaling diagram of a de-register procedure caused by aPS detach according to an embodiment of the present invention.

FIG. 5 shows a signaling diagram of a de-register procedure caused by aPDP context deactivation according to an embodiment of the presentinvention.

FIG. 6 shows a signaling diagram of a de-register procedure caused by PSdetach according to an embodiment of the present invention.

FIG. 7 shows a signaling diagram of a de-register or session terminationprocedure caused by an ICMP message according to a preferred embodimentof the present invention.

DESCRIPTION OF THE PRESENT INVENTION

FIG. 1 shows a signaling diagram, schematically illustrating a GPRSattach performed by a UE (User Equipment) towards a GPRS (General PacketRadio Service) network via a UTRAN (Universal Terrestrial Radio AccessNetwork). Before registering with an IM (IP Multimedia) network via aCSCF (Call Server Control Function), the UE has also to activate a PDPcontext towards a 3G-GGSN (Third Generation Gateway GPRS Support Node)via a 3G-SGSN (Third Generation Serving GPRS Support Node). The GGSNallocates to the UE an IP address which is used by the GPRS network toaddress the UE. This IP address is conveyed to the IM network during theapplication level registration by the UE or the User Application.

The UE may have multiple PDP Contexts activated with GPRS and havemultiple IP addresses in use. Moreover, the UE may be registered withmultiple CSCFs, where each CSCF may have multiple IP addresses for thesame user, depending on the application or service.

In order to solve the above-described problem of inconsistent IPaddresses used by the GPRS and the IM networks to address the users,arising due to the situation when an IP address assigned to a UE isde-allocated by the GPRS network for explicit or implicit reasonswithout the IM network knowing it, the IM network should be somehowinformed about the de-allocation of the IP address assigned to a UE. Asa minimal solution, the GPRS network must at least detect messages anddata intended for the UE which had been assigned the now de-allocated IPaddress and discard them.

In situations where IM network cannot be informed about the release orreallocation of a UE IP address and the SGSN/GGSN does not “sniff” themobile terminated call/session setups, the IM network must authenticatethe user in every mobile terminated transaction to make sure that it isstill the same user reached at the IP address which the CSCF hasregistered for a particular user. This, however, does not solve theproblem of signaling messages being sent to a wrong target, but reducesthe seriousness of the problem, as this would make sure that the mobileterminated setup fails due to authentication failure, though somesignaling takes place.

Moreover, the UEs may be required to perform periodic registrationupdates at certain time intervals as agreed by the UE and the IM networkduring the first registration. A registration performed with a CSCF inan IM network would terminate if the UE does not perform a periodicregistration update. This, however does not completely solve theabove-described problem since the UE may have a longer duration forperiodic registration updates than the one at the GPRS level forperiodic routing area update and the two procedures may not besynchronized. Further, the possibility of explicit GPRS leveldeactivation while there is a valid IM level registration would stillcause the above-mentioned problem.

These approaches merely reduce the chances or seriousness of theabove-described problem. In the following, embodiments of the presentinvention are described, which provide complete solutions of theproblem.

FIG. 2 shows a signaling diagram illustrating an IP addressde-registration according to an embodiment of the present invention.

In order to register for IM network services, the UE is required toselect a CSCF in the IM network with the assistance of the GPRS network,i.e. the GGSN, known as CSCF discovery procedure. Hence, the GGSN isaware of the CSCFs proposed to the UE. The GGSN also stores theaddresses of the proposed CSCFs against the allocated IP address of theuser and the IMSI (International Mobile Subscriber Identity), whichidentifies the user in the GPRS network. When the IP address of the useris de-allocated (FIG. 2: 1. IP address de-allocation) due to explicitreasons, i.e. when the UE performs a PDP context deactivation, or due toimplicit reasons, i.e. after the UE becomes unreachable or fails toperform a periodic routing area update, the GGSN informs about this toall the proposed CSCFs (communication 2 in FIG. 2: the 3G-GGSN transmitsa de-register message to the CSCFS), which were stored in the table.Thereupon, the CSCFs may acknowledge by sending a de-register Ackmessage (communication 3 in FIG. 2) to the GGSN. The CSCFs initiate thede-registration upon receiving the de-register message from the GGSN(FIG. 2: 4. De-registration).

In particular, in case of an explicit PDP context de-activation, the UEmay choose to deactivate just one or two PDP contexts while it continuesto have the rest of PDP contexts active. In this case, only the IPaddresses allocated by the deactivated PDP contexts will be de-allocatedwhile the UE continues to use other existing IP addresses. Thesede-allocated IP addresses may be used to address the UE by one or moreCSCFs. Hence, in this case, the GGSN notifies only the one or more CSCFsusing the de-allocated IP addresses to address the particular UE.

In the case of an implicit PDP context activation, the GPRS mayde-activate all the active PDP contexts and IP addresses for thatparticular UE. Hence, in this case, the GGSN notifies all CSCFs of theparticular UE for all IP addresses.

The GGSN must have up-to-date knowledge of the IP addresses used by a UEand the CSCFs the UE is registered to. Therefore, ever time the UEperforms a new registration or de-registration, the GGSN must be awareof it.

The disadvantage of this solution is that the CSCFs which were notselected by the UE will also be notified. This can be avoided byinforming the GGSN providing the addresses of the CSCFs (or logicalnames that can be used to get the actual address), whenever the UEregisters with one or more CSCFs for service. The GGSN stores theaddresses of the CSCFs against the allocated IP address of the user andthe IMSI. When the IP address of the user is de-allocated, the GGSNinforms about this to all those CSCFs, which were stored. When the UEde-registers with a CSCF too, it notifies the GGSN, so that the GGSN canremove this CSCF from the stored list. This way, only the CSCFs the UEhas actually been registered to are notified by the GGSN.

Moreover, the mobile-terminated call/session setup message from the CSCFcan be defined to contain also a user identifier like IMSI, which is thesubscriber identity at GPRS level. The GGSN checks all themobile-terminated call setup messages and compares them against its listof active IMSIs and the corresponding IP addresses. If the received IMSIand IP address pair is not found, the GGSN discards the data. Optionallythe GGSN could also notify the CSCF about the situation if the CSCFaddress is known.

FIG. 3 shows a signaling diagram illustrating a de-registrationprocedure due to a PDP context deactivation according to an embodimentof the present invention.

This embodiment utilizes a PCF (Policy Control Function) to relay theindication on IP address de-allocation from the access network to the IMnetwork. In general, the task of Policy Control is to divide theavailable network resource in a defined manner over the set of networkusers competing for that resource. Policy Control enforces a connectionadmission policy. A connection admission policy is a set of rulesdefining under what circumstances (other than the mere availability ofthe requested resource) the request for a logical connection is to beaccepted.

The PCF is able to communicate with the GGSN and the CSCF. (It may alsobe possible for the PCF functionality to be incorporated into the CSCFor GGSN.) The UE needs a PDP context in order to communicate with theproxy CSCF. The PCF is informed about deactivating the signaling PDPcontext. As it is shown in communication 1 in FIG. 3, the signaling PDPcontext is deactivated in the GPRS network. Thereupon, the GGSN sends aDelete Request State (Request Id) message to the PCF (communication 2 inFIG. 3). The PCF notices that the deactivated PDP context is a signalingPDP context. This may happen either by receiving an indication of thesignaling PDP context from the GGSN or by checking the request stateinformation in the PCF. In communication 3 in FIG. 3, the PCF sends aDe-register (Subscriber Id) message to the proxy CSCF. The PCF may alsosend the de-register message to a serving CSCF serving the IM networkapplication requested by the UE.

The proxy CSCF may acknowledge by sending a De-register Ack (SubscriberId) message to the PCF (communication 4 in FIG. 3). Then, the proxy CSCFinitiates de-registration (FIG. 3: 5. De-registration) by informing theHSS about the de-registration.

When the UE deactivates its PDP contexts, the SGSN knows about it andforwards information (about the latest deactivated PDP context) to theGGSN. The GGSN forwards the information to the PCF, and the PCF forwardsit to the CSCF.

The access network can also trigger de-registration from the IPmultimedia services when the UE performs a PS detach, i.e. when the IPaddress assigned to the UE is de-allocated implicitly as mentionedabove. This case is shown in FIG. 4, illustrating a de-registration dueto PS detach according to an embodiment of the present invention. Incase of a PS detach (FIG. 4: 1. PS detach), the SGSN informs an HLR(Home Location Register) by performing the Purge procedure(communication 2 in FIG. 4). The HLR in turn informs a UMS (UserMobility Server) about the PS detach (communication 3 in FIG. 4), whichthen informs the CSCF (communication 4 in FIG. 4). When receiving theinformation, the CSCF initiates de-registration (communication 4 in FIG.4). The CSCF may send a De-register Ack message (communication 5 in FIG.4) to the UMS.

FIG. 5 shows a signaling diagram illustrating a de-registration due to aPDP context deactivation according to an embodiment of the presentinvention. When the PDP context is deactivated in the GPRS network, theGGSN indicates to an HSS (Home Subscriber Server) about it (FIG. 5: 2.IP address de-allocation). The HSS knows all possible CSCFs the UE hasbeen registered with, and therefore, it informs the relevant CSCFs insending de-register messages (communication 3 in FIG. 5). The relevantCSCFs may send de-register Ack messages to the HSS and initiatede-registration.

In the case of an explicit PDP context deactivation, the GGSN notifiesthe HSS about the particular UE and only the de-allocated IP addressesand then the HSS updates its own record and notifies only the one ormore CSCFs using the deallocated IP addresses to address the particularUE.

FIG. 6 shows the case of a de-registration due to a PS detach accordingto this embodiment. For example, if the UE fails to perform the periodicrouting area update, the SGSN informs the HSS about it by performing thePurge procedure (communication 2 in FIG. 6), and the HSS in turnnotifies the CSCFs serving the UE by sending the de-register message(communication 3 in FIG. 6). The relevant CSCFs may send the de-registerAck message and initiate the de-registration.

In the case of an implicit PDP context deactivation, the GPRS networkde-activates all the active PDP contexts and IP addresses for thatparticular UE. Hence, the de-registration performed by the HSS is donefor all IP addresses and all CSCFs of the particular UE.

The HSS must have up-to-date knowledge of the IP addresses used by a UEand the CSCFs the UE is registered to. Therefore, ever time the UEperforms a new registration or de-registration, the HSS must be aware ofit.

FIG. 7 shows a signaling diagram according to the preferred embodimentof the present invention.

As described above, in certain cases, e.g. in error cases, it ispossible that all the PDP contexts of a UE are deactivated but the UEstill remains registered to the IM network. In this case, it is notpossible to carry a MT (Mobile Terminated) SIP (Session InitiationProtocol) message to the UE even if the IM network thinks that the UE isregistered.

As shown in FIG. 7, if the GGSN receives a SIP message (communication 1in FIG. 7) and does not have an active PDP context with the IP addressindicated in the SIP message (block 2 in FIG. 7), then the GGSN sends anICMP (Internet Control Message Protocol) message ‘Host not reachable’ tothe node which initiated the SIP message, i.e. to the P-CSCF(communication 3 in FIG. 7).

In block 2 in FIG. 7, the GGSN determines whether or not the IP addressindicated in the received SIP message belongs to an active PDP context.If there is no active PDP context with this IP address, the GGSN issuesthe ICMP message to the P-CSCF.

It is to be noted that a PCF can be connected between the GGSN which maybe a 3G-GGSN and the P-CSCF in a manner similar as shown in FIG. 3.

The P-CSCF can then either terminate the session which is under thesetup phase or terminate all active sessions or initiate de-registrationof the UE (block 4 in FIG. 7).

Session termination requires sending a reject message or a BYE messagefrom the P-CSCF towards the peer. If session termination is performed, aflag may be set in the IM network, e.g. in the P-CSCF or S-CSCF, toindicate that the UE is not anymore available for MT SIP messages. TheUE is thus kept registered, but no MT SIP messages are sent to the UEbefore the UE becomes available again. The IM network knows that the UEis available again e.g. when the UE performs re-registration. If the UEis able to perform re-registration, it has an active PDP context tocarry SIP messages. Thus, e.g. at re-registration, the flag in the IMnetwork can be removed. This also indicates that if the UE has lost allits PDP contexts, it should perform re-registration immediately whengetting an active PDP context.

It is to be noted that a flag indicating that the user equipment is notavailable for user equipment terminated messages may be set not onlywhen session termination is performed. For example, such flag may be setwhen a message is received which indicates that there is no active PDPcontext with the specific IP address.

De-registration may require sending a SIP message from the P-CSCF to theS-CSCF, e.g. a 480 Temporarily Unavailable message with an indication toperform de-registration, and initiate de-registration from the S-CSCF.As an alternative, the P-CSCF may initiate de-registration immediately.The latter is not the preferred solution, because currently the P-CSCFis not allowed to perform de-registration.

It would be good to know in the IM network if the UE has becomeunavailable. This way, SIP messages would not be sent to such a UE,because the UE is not able to receive them.

Though the invention is described referring to GPRS as the wirelessaccess network, the principles of the invention can be applied also toother wireless networks.

According to one aspect, a communication network system comprising atleast one first network (GPRS) and at least one second network providingapplication services over the first network is described. The firstnetwork allocates an IP address used by the second network. The systemcomprises means for detecting that the IP address allocated to a userequipment is de-allocated by the first network and means for initiatinga de-registration of the de-allocated IP address in the second network.

While the invention has been described with reference to a preferredembodiment, the description is illustrative of the invention and is notto be construed as limiting the invention. Various modifications andapplications may occur to those skilled in the art without departingfrom the true spirit and scope of the invention as defined by theappended claims.

1. A system, comprising: at least one first network and at least onesecond network providing application services over the first network,the first network allocating an IP address of a user equipment based ona communication channel activated by the user equipment in the firstnetwork, the IP address being conveyed to the second network during anapplication level registration at the second network by the userequipment; a first network unit configured to receive a messageassociated with said IP address from the second network, configured todetect that there is no active communication channel for said IPaddress, and configured to send an indication that the user equipment isnot reachable through said IP address to the second network; and asecond network unit configured to receive said indication, wherein thesecond network unit, upon receiving the user equipment not reachableindication, is configured to initiate a de-registration of the userequipment in the second network, and wherein the second network unit,for initiating the de-registration, is configured to send a message withan indication to perform de-registration to a serving network node inthe second network, said message being sent to inform at least one othernetwork element that said IP address can no longer be used to reach saiduser equipment.
 2. The system according to claim 1, wherein said messagecomprises the IP address.
 3. The system according to claim 1, whereinthe second network comprises an inhibiting unit configured to inhibitsending further messages to the unreachable user equipment uponreceiving the user equipment not reachable indication sent from thefirst network means.
 4. The system according to claim 3, wherein thesecond network unit, upon receiving the user equipment not reachableindication, is configured to set a flag indicating that the userequipment is not available for user equipment terminated messages. 5.The system according to claim 4, wherein the second network unit isconfigured to remove the flag when the user equipment performsre-registration towards the second network.
 6. The system according toclaim 1, wherein the second network unit, upon receiving the userequipment not reachable indication, is configured to terminate theinvited session by sending a reject message to a peer entity.
 7. Thesystem according to claim 1, wherein the second network unit, uponreceiving the user equipment not reachable indication, is configured toterminate all active sessions.
 8. The system according to claim 1,wherein the second network unit is configured to initiate thede-registration directly.
 9. The system according to claim 1, whereinthe first network unit is configured to send the user equipment is notreachable indication to the node in the second network, which initiatedsaid message.
 10. The system according to claim 1, wherein the userequipment is not reachable indication is an Internet control messageprotocol message.
 11. The system according to claim 1, wherein the firstnetwork is a wireless access network.
 12. The system according to claim1, wherein the second network is an IP multimedia network.
 13. Thesystem according to claim 1, wherein the communication channel is apacket data protocol context.
 14. A method, comprising: receiving amessage associated with an IP address from at least one second networkin a first network unit, the IP address being allocated by the firstnetwork to a user equipment based on a communication channel activatedby the user equipment in the first network and conveyed to the secondnetwork during an application level registration at the second networkby the user equipment, wherein the second network provides applicationservices over the first network; detecting in the first network unitthat there is no active communication channel for said IP address;sending an indication that the user equipment is not reachable throughsaid IP address from the first network unit to the second network;receiving said indication by a second network unit; and upon receivingthe user equipment not reachable indication, initiating ade-registration of the user equipment in the second network, wherein theinitiating comprises sending a message with an indication to performde-registration to a serving network node in the second network, saidmessage being sent to inform at least one other network element thatsaid IP address can no longer be used to reach said user equipment. 15.The method according to claim 14, wherein said message comprises the IPaddress.
 16. The method according to claim 14, further comprisinginhibiting, by the second network unit, sending further messages to theunreachable user equipment upon receiving the user equipment notreachable indication sent from the first network unit.
 17. The methodaccording to claim 16, upon receiving the user equipment not reachableindication, further comprising setting a flag in the second network,which indicates that the user equipment is not available for userequipment terminated messages.
 18. The method according to claim 17,wherein the flag is set in the second network unit.
 19. The methodaccording to claim 17, wherein the flag is set in a serving networknode.
 20. The method according to claim 17, wherein the flag is removedwhen the user equipment performs re-registration towards the secondnetwork.
 21. The method according to claim 14, upon receiving the userequipment not reachable indication, comprising terminating the invitedsession by sending a reject message from the second network unit to apeer entity.
 22. The method according to claim 14, upon receiving theuser equipment not reachable indication, comprising terminating allactive sessions.
 23. The method according to claim 14, wherein thede-registration is performed by the second network unit directly.
 24. Asystem, comprising: at least one first network and at least one secondnetwork providing application services over the first network, the firstnetwork allocating an IP address to a user equipment based on acommunication channel activated by the user equipment in the firstnetwork, the IP address being conveyed to the second network during anapplication level registration at the second network by the userequipment; a detector configured to detect that the IP address isde-allocated by the first network; an initiating unit configured toinitiate a de-registration of the de-allocated IP address in the secondnetwork; and a control entity configured to communicate with a gatingnetwork element in the first network and a dedicated call servingnetwork element in the second network, the dedicated call servingnetwork element serving the requested application, wherein the gatingnetwork element detects a communication channel deactivation, andinforms the control entity thereof, and wherein the control entitydetects that the deactivated communication channel represents asignaling packet data protocol context and initiates the de-registrationin the dedicated call serving network element by sending a message, saidmessage being sent to inform at least one other network element thatsaid IP address can no longer be used to reach said user equipment. 25.The system according to claim 24, wherein the gating network elementsupplies an indication of the signaling packet data protocol context tothe control entity.
 26. The system according to claim 24, wherein thecontrol entity detects that the deactivated communication channelrepresents a signaling packet data protocol context by checking requeststate information in the control entity.
 27. The system according toclaim 24, wherein the gating network element is informed by a servingnetwork element in the first network about a communication channeldeactivation and forwards this information to the control entity. 28.The system according to claim 24, wherein a network element in the firstnetwork detects the de-allocation of the IP address and informs a serverabout the de-allocation, and the server initiates de-registration incall serving network elements in the second network, to which saidIP-address is associated with.
 29. The system according to claim 28,wherein the server is a home subscriber server.
 30. The system accordingto claim 28, wherein the network element in the first network informs adatabase about the de-allocation, a home database informs a mobilityserving entity about the de-allocation, and the mobility serving entityinitiates the de-registration in a dedicated call serving networkelement in the second network, the dedicated call serving networkelement serving the requested application.
 31. The system according toclaim 28, wherein the network element in the first network detects adetach of the user equipment causing the de-allocation for all IPaddresses of the user equipment and informs the server about thede-allocation, and the server initiates the de-registration in all callserving network elements of the user equipment.
 32. The system accordingto claim 28, wherein a gating network element in the first networkstores addresses of call serving network elements proposed to the userequipment against the allocated IP address and a user identification,the gating network element allocating the IP address and being involvedin a discovery procedure of one of the call serving network elements inthe second network, the call serving network elements being able toserve the requested application, and, when the gating network elementdetects the de-allocation of the IP address, it initiatesde-registration in all stored call serving network elements associatedwith the IP addresses in the second network.
 33. The system according toclaim 28, wherein the user equipment informs a gating network element inthe first network whenever it registers with a call serving networkelement in the second network, the gating network element allocating theIP address and being involved in the discovery procedure of the callserving network element in the second network, the call serving networkelements being able to serve the requested application, wherein thegating network element stores addresses of the registered call servingnetwork elements against the allocated IP addresses and a useridentification, and, when the gating network element detects thede-allocation of the IP addresses, it initiates the de-registration inall stored call serving network nodes associated with the IP addresses.34. The system according to claim 28, wherein a de-allocation of the IPaddress caused by a communication channel deactivation is detected by agating network element in the first network, and the server is informedby the gating network element about the de-allocation.
 35. The systemaccording to claim 28, wherein a de-allocation of the IP address causedby a detach of the user equipment is detected by a serving networkelement in the first network, and a home server is informed by theserving network element about the de-allocation.
 36. The systemaccording to claim 28, wherein the server is informed by the userequipment about all the call serving network elements the user equipmentis registered with.
 37. The system according to claim 24, wherein the IPaddress is de-registered in the second network by deleting a record in ahome server in the first network by a call serving network element inthe second network.
 38. The system according to claim 24, wherein an IPaddress is allocated for each communication channel request performed bythe user equipment.
 39. The system according to claim 24, wherein theinitiation of the de-registration comprises the supply of a de-registermessage.
 40. The system according to claim 24, wherein the first networkis a wireless access network.
 41. The system according to claim 24,wherein the second network is an IP multimedia network.
 42. The systemaccording to claim 24, wherein the communication channel is a packetdata protocol context.
 43. An apparatus, comprising: a receiverconfigured to receive information from a gating network element in afirst network about a communication channel deactivation, wherein theapparatus communicates with a dedicated call serving network element ina second network which provides application services over the firstnetwork, the first network allocates an IP address to a user equipmentbased on a communication channel activated by the user equipment in thefirst network, the IP address being conveyed to the second networkduring an application level registration at the second network by theuser equipment, and the dedicated call serving network element serves arequested application; and a detector configured to detect that thedeactivated communication channel represents a signaling packet dataprotocol context and initiates the de-registration in the dedicated callserving network element by sending a message to inform at least oneother network element that said IP address can no longer be used toreach said user equipment.
 44. A method, comprising: detecting that anIP address allocated to a user equipment is de-allocated by at least onefirst network, wherein the first network communicates with at least onesecond network that provides application services over the firstnetwork, and the first network allocates the IP address to the userequipment based on a communication channel activated by the userequipment in the first network, the IP address being conveyed to thesecond network during an application level registration at the secondnetwork by the user equipment; initiating a de-registration of thede-allocated IP address in the second network; providing a controlentity being arranged to communicate with a gating network element inthe first network and a dedicated call serving network element in thesecond network; and detecting at the dedicated call serving networkelement serving the requested application a communication channeldeactivation by the gating network element and informing the controlentity thereof, wherein when it is detected by the control entity thatthe deactivated communication channel represents a signaling packet dataprotocol context, the de-registration in the dedicated call servingnetwork element is initiated, and wherein the de-registration isinitiated by sending a message to inform at least one other networkelement that said IP address can no longer be used to reach said userequipment.
 45. The method according to claim 44, wherein an indicationof the signaling packet data protocol context is supplied to the controlentity by the gating network element.
 46. The method according to claim44, wherein it is detected by the control entity that the deactivatedcommunication channel represents the signaling packet data protocolcontext by checking request state information in the control entity. 47.The method according to claim 44, wherein the gating network element isinformed by a serving network element in the first network about thecommunication channel deactivation and this information is forwarded tothe control entity.
 48. The method according to claim 44, wherein thede-allocation of the IP address is detected by a network element in thefirst network and a server is informed about the de-allocation, andde-registration is initiated in call serving network elements in thesecond network, to which said JP-address is associated with.
 49. Themethod according to claim 48, wherein the server is a home subscriberserver.
 50. The method according to claim 48, wherein a database isinformed by the network element in the first network about thedc-allocation, a mobility serving entity is informed by a home databaseabout the de-allocation, and the de-registration is initiated by themobility serving entity in a dedicated call serving network element inthe second network, the dedicated call serving network element servingthe requested application.
 51. The method according to claim 48, whereina detach of the user equipment causing the de-allocation for all IPaddresses of the user equipment is detected by the network element inthe first network and the server is informed about the de-allocation bythe network element, and de-registration in all call serving networkelements of the user equipment is initiated by the server.
 52. Themethod according to claim 48, wherein addresses of call serving networkelements proposed to the user equipment against the allocated IP addressand a user identification are stored in a gating network element in thefirst network, the gating network element allocating the IP address andbeing involved in the discovery procedure of a call serving networkelement in the second network, the call serving network elements beingable to serve the requested application, and, when the gating networkelement detects the dc-allocation of the IP address, de-registration inall stored call serving network elements associated with the IPaddresses in the second network is initiated by the gating networkelement.
 53. The method according to claim 48, wherein a gating networkelement in the first network is informed by the user equipment wheneverit registers with one of the call serving network elements in the secondnetwork, the gating network element allocating the IP address and beinginvolved in the discovery procedure of said one of the call servingnetwork elements in the second network, the call serving networkelements being able to serve the requested application, wherein thegating network element stores addresses of the registered call servingnetwork elements against the allocated IP addresses and a useridentification, and, when the gating network element detects thede-allocation of the IP addresses, it initiates the de-registration inall stored call serving network nodes associated with the IP addresses.54. The method according to claim 48, wherein a de-allocation of the IPaddress caused by a communication channel deactivation is detected by agating network element in the first network, and the server is informedby the gating network element about the de-allocation.
 55. The methodaccording to claim 48, wherein a de-allocation of the IP address causedby a detach of the user equipment is detected by a serving networkelement in the first network, and the home server is informed by theserving network element about the de-allocation.
 56. The methodaccording to claim 48, wherein the server is informed by the userequipment about all call serving network elements the user equipment isregistered with.
 57. The method according to claim 44, wherein the IPaddress is de-registered in the second network by deleting a record in ahome server in the first network by a call serving network element inthe second network.
 58. The method according to claim 44, wherein an IPaddress is allocated for each communication channel request performed bythe user equipment.
 59. The method according to claim 44, wherein theinitiation of the de-registration comprises the supply of a de-registermessage.
 60. The method according to claim 44, wherein the first networkis a wireless access network.
 61. The method according to claim 44,wherein the second network is an IP multimedia network.
 62. The methodaccording to claim 44, wherein the communication channel is a packetdata protocol context.
 63. A system, comprising: at least one firstnetwork and at least one second network providing application servicesover the first network, the first network allocating an IP address to auser equipment based on a communication channel activated by the userequipment in the first network, the IP address being conveyed to thesecond network during an application level registration at the secondnetwork by the user equipment; first network means for receiving amessage associated with said IP address from the second network, fordetecting that there is no active communication channel for said IPaddress, and for sending an indication that the user equipment is notreachable through said IP address to the second network; and secondnetwork means for receiving said indication, wherein the second networkmeans, upon receiving the user equipment not reachable indication,initiates a de-registration of the user equipment in the second network,and wherein the second network means, for initiating thede-registration, sends a message with an indication to performde-registration to a serving network node in the second network, saidmessage being sent to inform at least one other network element thatsaid IP address can no longer be used to reach said user equipment. 64.A system, comprising: at least one first network and at least one secondnetwork providing application services over the first network, the firstnetwork allocating an IP address to a user equipment based on acommunication channel activated by the user equipment in the firstnetwork, the IP address being conveyed to the second network during anapplication level registration at the second network by the userequipment; detecting means for detecting that the IP address isde-allocated by the first network; initiating means for initiating ade-registration of the de-allocated IP address in the second network;and control entity means for communicating with a gating network elementin the first network and a dedicated call serving network element in thesecond network, the dedicated call serving network element serving therequested application, wherein the gating network element detects acommunication channel deactivation, and informs the control entity meansthereof, and wherein the control entity means detects that thedeactivated communication channel represents a signaling packet dataprotocol context and initiates the de-registration in the dedicated callserving network element by sending a message to inform at least oneother network element that said IP address can no longer be used toreach said user equipment.
 65. An apparatus, comprising: receiving meansfor receiving information from a gating network element in a firstnetwork about a communication channel deactivation, wherein theapparatus communicates with a dedicated call serving network element ina second network which provides application services over the firstnetwork, the first network allocates an IP address to a user equipmentbased on a communication channel activated by the user equipment in thefirst network, the IP address being conveyed to the second networkduring an application level registration at the second network by theuser equipment, and the dedicated call serving network element serves arequested application; and detecting means for detecting that thedeactivated communication channel represents a signaling packet dataprotocol context and initiates the de-registration in the dedicated callserving network element by sending a message to inform at least oneother network element that said IP address can no longer be used toreach said user equipment.
 66. An apparatus, comprising: a receiverconfigured to receive a message associated with an IP address from asecond network which uses the IP address and provides applicationservices over the first network, the IP address being allocated by afirst network to a user equipment based on a communication channelactivated by the user equipment in the first network, the IP addressbeing conveyed to the second network during an application levelregistration at the second network by the user equipment, a detectorconfigured to detect that there is no active communication channel forsaid IP address, and a transmitter configured to send an indication thatthe user equipment is not reachable through said IP address to thesecond network said indication comprising a message that informs atleast one other network element that said IP address can no longer beused to reach said user equipment.
 67. An apparatus, comprising: atransmitter configured to send a message associated with an IP addressfrom a second network, which uses the IP address and providesapplication services over a first network, to the first network, the IPaddress being allocated by the first network to a user equipment basedon a communication channel activated by the user equipment in the firstnetwork, the IP address being conveyed to the second network during anapplication level registration at the second network by the userequipment, a receiver configured to receive an indication that the userequipment to which the IP address is allocated is not reachable throughsaid IP address, and an initiating unit configured to, upon receivingthe user equipment not reachable indication, initiate a de-registrationof the user equipment in the second network, wherein, for initiating thede-registration, the initiating unit is configured to send a messagewith an indication to perform de-registration to a serving network nodein the second network by sending a message to inform at least one othernetwork element that said IP address can no longer be used to reach saiduser equipment.